Generally, a capacitive microphone utilizes a capacitance between a pair of electrodes which are facing each other to generate an acoustic signal. A MEMS microphone may be manufactured by a semiconductor MEMS process to have an ultra-small size.
A MEMS microphone may include a bendable diaphragm and a back plate which is facing the diaphragm. The diaphragm can be a membrane structure to generate a displacement due to the acoustic pressure. In particular, when the acoustic pressure reaches to the diaphragm, the diaphragm may be bent upwardly or downwardly due to the acoustic pressure. The displacement of the diaphragm can be sensed through a change of capacitance between the diaphragm and the back plate. As a result, an acoustic wave can be converted into an electrical signal to for output.
The MEMS microphone may include an anchor which may spaces the diaphragm from the substrate so that the diaphragm can be freely bent by the acoustic pressure, and a strut that spaces the back plate from the diaphragm. The anchor and the strut serve as columns for supporting the diaphragm and the back plate, respectively, and each of lower portions thereof is coupled to the upper surface of the substrate.
However, since the anchor and the strut are coupled to the substrate with a structure in which each of the lower portions thereof is simply bonded to the upper surface of the substrate without a separate coupling structure, the coupling force with which the anchor and the strut are coupled to the substrate is inevitably weak.
In particular, the above-mentioned structural weakness where the anchor and strut are bonded with the substrate can reduce a reliability of the anchor and the strut. That is, when the MEMS microphone is driven, the diaphragm is frequently bent or deformed by the applied acoustic pressure. The movement of the diaphragm may affect the anchor, especially a bonding portion where the anchor is bonded to the substrate. As a result, the coupling force between the diaphragm and the substrate may be deteriorated. Furthermore, as the deformation of the diaphragm is been repeated many times, the coupling force between the anchor and the substrate gradually weakens, so that the bonding portion of the anchor can be lifted or separated from the substrate. In addition, the movement of the diaphragm can affect the back plate, and thus, a movement of the back plate may affect the strut, which may cause the coupling force between the back plate and the substrate to deteriorate. As a result, the coupling force between the strut and the substrate is lowered, and a bonding portion where the strut is bonded to the substrate may be lifted or separated from the substrate.
In addition, it may be difficult to secure the stability of the process of removing the insulation layer around the diaphragm during the MEMS microphone manufacturing process. That is, in the process of forming the anchor and the strut, a portion of the insulation layer located where the anchor and/or the strut are to be formed may not be completely removed. While removing the residual portion of the insulation layer from the substrate, the anchor and/or the strut may be lifted or separated from the substrate.
The characteristics of a MEMS microphone can be determined by measuring various factors such as a pull-in voltage, sensitivity, a frequency resonance, and a total harmonic distortion (THD). In particular, a THD value may serve as an indicator for or a symptom of unnecessary harmonic components in the acoustic signal to result in distortion. The distortion component is a component tone which is not originally included in the input acoustic signal, which may cause sound quality to degrade upon reproduction. Therefore, the MEMS microphone should ideally be configured such that the THD value does not exceed a proper level. THD values can be is relatively high in cases where the flexibility of the diaphragm is low.
One conventional method for increasing the flexibility of the diaphragm includes forming a plurality of slits in the diaphragm. However, such conventional methods result in lowering the rigidity of the diaphragm. Therefore, in order to improve the quality of the MEMS microphone, both the stiffness and the flexibility of the diaphragm should be required to be maintained.